Transmission chains known as “seal chains” have been used for power transmission, and other applications such as conveyor applications, in dusty atmospheres. In a seal chain, seals are disposed between the inner and outer plates of the chain in order to prevent leakage of lubricant from, and to prevent entry of dust and other foreign matter into, the load-bearing interface between outer circumferential surfaces of the connecting pins of the chain and the inner circumferential surfaces of the bushings through which the connecting pins extend.
Annular elastic seals such as O-rings have been used. An improved seal, as shown in FIG. 5, is disclosed in laid-open Japanese Patent Application No. 2005-076681. The improved seal comprises a pair of annular seal members 531 and 532 facing each other between an inner plate 511 and an outer plate 521 of a chain. The facing surfaces of the annular seal members 531 and 532 have annular lip-like portions 531a and 532a and annular recesses 531b and 532b. Edges of the annular lip-like portions 531a and 532a are pressed by the elasticity of the seal members 531 and 532 against the bottoms of the annular recesses 531b and 532b so that the edges of the lip-like portions 531a and 532a slide against the bottoms of the annular recesses 531b and 532b when the seal chain bends.
In another seal mechanism, disclosed in laid-open Japanese Patent Application No. 2008-157423 and shown in FIG. 6, an annular seal-receiving member 632, composed of steel is fitted on and fixed to a connecting pin 622, and contacts an inner surface of an outer plate 621 of a chain. A disc-shaped part 632a of the seal-receiving member is interposed between an end surface of a bushing 612 and the inner surface of the outer plate 621 and having a disc-like seal receiving portion 632a. A rim 632b on the outer circumferential part of the seal-receiving member 632 extends toward an inner plate 611. A first elastic seal ring 631, fitted to the outer circumferential surface of the bushing 612, is interposed between the annular seal-receiving member 632 and the inner plate 611. A seal ring 633, also composed of steel, disposed to the outside of the annular seal receiving member 632 contacts with the outer side surface of the inner plate 611, and a second elastic seal ring 634 is sandwiched between the inner circumferential surface of the seal ring 633 and the outer circumferential surface of the rim 632b. 
The seal mechanism of FIG. 5 comprises a large number of parts and requires time-consuming adjustment of the relative positions of the annular seal members 531 and 532 when inserting the lip-like portions 531a and 532a into the annular recesses 531b and 532b. The positions of the seal members, and of the ring member which fits loosely around the outer circumferences of the seal members, are not stable, and consequently there is a possibility of leakage of lubricant through paths between the seal members and the plates on which they are disposed.
In the seal mechanism of FIG. 6, the annular seal-receiving member 632 is pressed tightly against the inner surface of the outer plate 621 by the elastic seal ring 634. However, ring 634 is not fixed either to ring 633 or to seal-receiving member 632. Lubricant adheres to the elastic seal ring 632, and, as the ring 634 wears, its sealing performance deteriorates as it slides relative to the ring 633 and the seal-receiving member 632.
Furthermore, because the annular seal-receiving member 632 can slide on the inner surface of outer plate 621, the inner surface of plate 621 can wear and lose strength.